The way Elbow says to do it is to start writing and keep writing.Only at the end will you know what you are saying, says he.
The problem is anxiety is trying to get you stuck and disgusted and to stop writing altogether. It is writing that causes all the anxiety.
Write until you get tired enough to drift, wander, and digress. You may have to write a manuscript as long as the thing you want to end up with before you write a paragraph that seems decent or a bit interesting. Producing writhing may be less like filling a basin and more like getting water to flow through it.
The idea may not be to begin at one end and write until you get to the other. The idea may be to doodle 'till you produce a rough sketch and then to make successive sketches that get clearer and more detailed, and more accurate. Seems a holistic activity.
You may feel stuck and frustrated and unable to go on. You realize that you have a problem. You may do some analysis and begin to see the problem more clearly and the solution too. You may just come to know that if you forge bravely on, you will eventually com up with a decent piece of writing.
There are five chapters in Peter's book and we are only half way through chapter two.